The present disclosure relates to a variable focal length lens system suitable, for example, for a digital video camera, a digital still camera, etc. and to an image pickup unit that uses such a variable focal length lens system. In particular, the present disclosure is suitable for a variable focal length lens system that has an angle of view covering an angle of view from about 14 mm to about 16 mm in 35 mm conversion in wide end state, an open F-value from about 2.8 to about 4, and a zoom ratio of about 2.
As a recording method used in a camera, a method in which a subject image is formed on a plane of an image pickup device that uses a photoelectric conversion device such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), and a light amount of the subject image is converted into an electric output by each photoelectric conversion device to be recorded is known.
On the other hand, in accordance with recent progress in microfabrication technology, high speed in central processing units (CPUs), high integration in recording media, etc. have been achieved. Accordingly, high-capacity image data that could not have been processed has been allowed to be processed at high speed. Further, high integration and reduction in size have been achieved also in the photoelectric conversion devices. The high integration in the photoelectric conversion devices has allowed recording at higher spatial frequency, and reduction in size of the photoelectric conversion devices has led to reduction in size of a camera as a whole.
However, due to the above-described high integration and reduction in size, a light receiving area of the individual photoelectric conversion device becomes smaller, which causes an issue that influence of noise becomes larger in accordance with decreasing electric output. In order to prevent this issue, an amount of light that arrives at the photoelectric conversion device has been increased by increasing aperture ratio of the optical system. Further, a fine lens device (a so-called micro-lens array) has been arranged just before each photoelectric conversion device. This micro-lens array guides light fluxes that arrive between adjacent devices onto the device. Instead of this, however, the micro-lens array limits a position of an exit pupil of the lens system. When the position of the exit pupil of the lens system becomes closer to the photoelectric conversion device, in other words, when an angle formed by the optical axis and a main light beam that arrives at the photoelectric conversion device becomes large, an off-axial luminous flux traveling toward periphery of the screen forms a large angle with respect to the optical axis. As a result, the off-axial luminous flux does not arrive at the photoelectric conversion device, which leads to insufficiency in light amount.
In a zoom lens that covers the range of the wide angle of view, a lens group having negative refractive power is often arranged at a most-object-sided position. More specifically, a so-called negative-positive two-group zoom lens is known that includes a first lens group having negative refractive power and a second lens group having positive refractive power in order from the object plane (for example, see Japanese Unexamined Patent Application Publication No. 2004-21223). Further, a so-called negative-positive-negative-positive four-group zoom lens is known that includes a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, and a fourth lens group having positive refractive power in order from the object plane (for example, see Japanese Unexamined Patent Application Publication Nos. 2010-249959, 2006-39531, and 2004-240038).
However, in the negative-positive two-group zoom lens, the number of movable lens group is small, and therefore, a lens diameter of the first lens group is likely to be large. On the other hand, in the negative-positive-negative-positive four-group zoom lens, the number of movable lens group is larger, and therefore, the lens diameter of the first lens group is allowed to be small. However, in the negative-positive-negative-positive four-group zoom lens, for example, as in Japanese Unexamined Patent Application Publication No. 2010-249959 described above, the first lens group is divided into two blocks, and a lens block arranged closer to the image plane in the first lens group is moved as a focus lens when a subject distance is varied. Therefore, a lens diameter of the focus lens is large, which causes difficulty in driving the focus lens. Moreover, the lens diameter of the focus lens is likely to be large since spaces for the focus lens to move are provided on the image plane side and on the object plane side of the focus lens.